Rubber base composite propellant and method of use



Sept. 10, 1968 G. H. SWART 3,400,539

RUBBER BASE COMPOSITE PROPELLANT AND METHOD OF USE Filed Jan. 14, 1954 Fig. L"

INVENTOR GILBERT HHSH/ VRTI DSC54550,

77/6 G-Efl/ERAL TI fE 4/7 #05551? 6041544); 435161755 ATTORNEY 3,400,539 RUBBER BASE COMPOSITE PROPELLANT AND METHOD OF USE Gilbert H. Swart, Akron, Ohio, assignor to The General Tire and Rubber Company, Akron, Ohio, at corporation of Ohio Filed Jan. 14, 1954, Ser. No. 404,014 9 Claims. (Cl. 60--219) ABSTRACT OF THE DISCLOSURE A solid rocket fuel is prepared by forming a liquid mixture which contains a synthetic rubber of a selected type e.g., polysulfide rubber, with a solid inorganic oxidizing agent e.g., potassium perchlorate, introducing the mixture into a mold and curing it until solid.

This application is a continuation-in-part of my application Ser. No. 100,785, filed June 23, 1949 which application is in turn a continuation-in-part of application Ser. No. 626,356, filed Nov. 2, 1945, which applications are now abandoned. This invention relates to a method of improving the performance of jets or rocket-s of the type used for vehicle-propulsion or braking. The invention particularly relates to such jets or rockets utilizing a nonliquid fuel and to a fuel suitable therefor.

The use of rockets or jets of high velocity gases for assisting in the take-off of airplanes and the like has reduced the safe take-off time or has permitted airplanes to be safely loaded to a considerably higher degree than before permissible. One type of jet utilizing solid fuel, such as asphalt or a heavy, meltable hydrocarbon mixed with an oxidizing agent was widely used. Such jets generally have a tubular chamber closed on one end and integrally connected at the other end to a nozzle. The chamber serves both as a combustion chamber and as a storage for the solid fuel used in the jet, which at the start of the burning operation occupies the major portion of the chamber. The surface of the fuel nearest the nozzle is first ignited and the fuel progressively consumed, until the opposite end of the chamber is reached. This process ordinarily consumes about 10 to 50 seconds or so.

For the successful operation of such rockets, it is generally considered essential that the burning always be confined to a plane nearest the nozzle portion and perpendicular to the longitudinal axis of the tubular chamber. If cracks or porosities of appreciable magnitude should occur Within the solid fuel, or if an additional surface of a fuel carrying its own oxygen should become subject to contact with the burning gases, the burning will thereafter continue from a surface area greater than that for which the jet was designed, with the result that explosions occur from the increased pressure produced.

Rockets or jets heretofore produced, as far as I am aware, have been subject to great disadvantage in that the fuel contained in the storage portion of the combustion chamber was not resistant to plastic flow and shrinkage cracks. Should the rockets be long exposed to a hot atmosphere, as for example the direct radiation from a summer sun, the solid fuel heretofore used would soften to such an extent that flow within the fuel would cause an increased surface area to be available for ignition, with the result that pressure built up beyond permissible limits and explosion occurred. On the other hand, should the rockets or jets be subjected to cold conditions, such as temperatures in the neighborhood of F., or below, shrinkage cracks frequently occurred which caused the same result. Such conditions have required rockets or jets to be stored under controlled conditions until just prior to use.

States Patent It is an object of the present invention to provide a vehicle-propelling rocket or jet of the type having selfcontained fuel, but which may be utilized with safety after indefinite storage in any habitable portion of the surface of the earth.

It is another object of the present invention to provide a vehicle-propelling rocket of the aforementioned type comprising a self-contained solid fuel as an integral portion thereof, which fuel has the solidity characteristics of fuel in a liquid state where air pockets are readily eliminated and later solidified to a nonplastic state.

It is another object of the present invention to provide a method of making vehicle-propelling jets wherein a resilient nonplastic fuel may be formed into the shape of a portion of the combustion chamber by a casting operation without added pressure and without causing any of the aforementioned disadvantages.

It is a further object of the present invention to provide a castable material suitable for forming the solid fuel of jets or rockets by simply treating the castable material with heat and/ or an appropriate catalyst.

It is a still further object of the present invention to provide a rocket fuel which may be cast to a desired shape and caused to solidify without retaining'stresses therein of a magnitude sufficient to produce cracks therein when subjected to temperatures in the neighborhood of -20 to 40 F.

These and other objects Will be apparent from the description herein as illustrated by the accompanying drawing, in which:

FIGURE 1 is a side elevational view, with parts broken away of a rocket or jet embodying the present invention; and

FIG. 2 is an enlarged sectional view through a portion of the fuel storage and combustion chamber of the rocket or jet of FIG. 1, showing the fuel in place.

In accordance with the present invention, the fuel portion of the rocket comprises a suitable oxidizing agent and combustible, nonplastic organic binding material which has been solidified from the viscous liquid (flowable) state by the application of heat and/or by the aid of suitable catalysts, etc., and which in the solid state retains rubbery properties or properties permitting flexure in the form of sheets having about /s" thickness at temperatures as low as 20 F., or in some cases as low as 40 F. or more.

The oxidizing agent is preferably a high oxygen-containing compound which exists in the solid state at normal temperatures. Solid inorganic oxidizing agents capable of being pulverized are usually most readily available, and of these potassium perchlorate, ammonium nitrate and ammonium perchlorate, etc. are most often used and are quite satisfactory. Other oxidizing agents, such as 2,2-dinitropropane, Tetryl, RDX and PETN, which are organic oxygen compounds, may also be used in combination with the above.

Refer-ring particularly to the drawings, wherein like parts are designated by like numerals of reference throughout the two views, the rockets or jets of the present invention have a nozzle 1, a combustion chamber 2 adjacent to and in communication with inner portion 3 thereof. The outer portion 10 of the nozzle is generally open to the atmosphere, but may carry suitable ignition means 11 which are adapted to be forced out of the nozzle upon ignition of the rocket. The combustion chamber 2 is enclosed within the metal shell 8, which carries the mounting means 9 for attaching to a vehicle, such as an airplane or the like. The chamber 2 is adapted to withstand heat and pressure and in the unfired rocket comprises a fuel storage portion which includes the end portion thereof farthest removed from the nozzle 1 and which contains the aforementioned solid fuel 4.

All surfaces of the fuel 4, with the exception of the face 5 which is nearest the nozzle 1 and which is preferably substantially in a plane perpendicular to the axis 6 of the tubular portion of the chamber 2, may be covered with an adherent layer 7 of a material which is incapable of combustion without oxygen supplied from an external source, so that burning at any instant will tend to be confined to successive surfaces in a plane generally parallel to the surface 5. The surface 5 of the unfired rocket is preferably spaced from the inner portion 3 of the nozzle to provide a chamber 2 of the minimum size required to permit combustion of gases vaporized from the burning surface and to permit adequate direction of the gases through the nozzle 1. After ignition of the fuel 4 at the surface 5, the burning surface gradually moves toward the opposite end of the chamber 2, so that at the end of period of effectiveness, the period wherein the rocket exerts a useful thrust, the entire chamber 2 serves as a combustion chamber.

In the manufacture of rocket fuels by masticating a solid oxidizing agent directly into a solid material, the elimination of entrapped air has been a major problem. Similarly, the direct mixing of solid rubbers with large amounts of inorganic or organic oxidizing agent has been considered dangerous. Rockets having certain solid rubbers as the sole binding ingredient for large amounts of powdered material also lack some of the desirable properties at low temperatures.

I have found that rockets or jets suitable for vehicleacceleration may be made by forming a relatively nonvolatile castable liquid comprising a polymeric material capable of being vulcanized to the solid state, similar to soft vulcanized rubber, by suitable treatment; incorporating a suitable solidification catalyst and an oxidizing agent (preferably a solid inoragnic oxidizing agent) into the organic liquid, which inorganic oxidizing agent is present in amounts at least equal to the weight of the total organic material in the propellant to provide the amount of oxygen required for combustion of said organic liquid; casting the mixture of organic material and oxidizing agent in the desired shape to occupy an end portion of the combustion chamber of the rocket or jet, and causing solidification of the mixture to the solid state. The fluid before solidification may for a time be subject to reduced pressure to facilitate removal of entrapped air, if desired. although this is usually unnecessary.

The fluid, castable, vulcanizable liquid may be a fluid polymer or copolymer of a rubber-forming material, such as one or more dienes, such as butadiene and including substituted butadienes, e.g. isoprene, chloroprene, dimethyl butadiene or methyl pentadiene, and other butadiene compounds, with or without other suitable copolymerizable unsaturated materials. The viscous castable, vulcanizable liquid is additionally characterized by substantial absence of readily volatile constituents, such as constituents which readily volatilize at temperatures below 100 C.

The preferred castable, vulcanizable liquids are prepared, however, by combining an unvulcanized organic rubbery material with a compatible liquid present in suflicient amount to peptize or disperse the rubbery material to the readily flowable state, such as generally characterized by rubber cements and the like. In the case of the fluid, castable partial polymers such as the fluid polymer of a butadiene compound, such as methyl pentadiene, etc., a mixture of polymers having different chain lengths are usually present. Some of the longer chain polymers are properly considered as rubbery materials and some of the shorter chain polymers are properly considered as liquids.

The rubbery material provides for the necessary resilience and flexibility of the product to prevent shrinkage cracks and fractures, as well as when vulcanized to prevent or reduce tendency for plastic flow. The solidification of the fuel mixture may be accomplished by vulcanization crosslinking of the rubbery or rubber-forming material.

Even when the rubbery or rubber-forming material is present in small amounts, amounts as low as 15 or 20% by weight of the total organic constituents, or even as low as 10% or so by weight of the organic constituents of the unsolidified fuel, rockets or jets having the desired properties are produced upon vulcanization of the rubbery material or upon solidification of the liquid constituent to the solid nonplastic state, so that the burnable mixture is in the solid state in the completed rocket. The amount of rubbery material present should not exceed that which permits the attainment of a liquid mixture readily flowable with or without pressure when combined with the organic liquid component. The maximum amount usable depends on the characteristics of the particular polymer, such for example as the length of the polymer chains, etc. Short chain polymers may be present in greater amounts. Generally the rubbery component should not greatly exceed 50 or 60% of the total organic constituents and in the case of rubbery diene and olefinic polymers and copolymers less than 35 or 40 percent is usually preferred.

The rubbery material may consist in whole or in part of a combustible rubbery polyolefin, such as polyisobutylene, polymers of one or more conjugated diolefinic compounds of less than 8 carbon atoms, such as butadiene compounds including butadine-1,3, isoprene, chloroprene, homologues and analogues thereof, a copolymer of one or more of conjugated dienes with one or more copolymerizable compounds containing a single active olefin group, including such monenes as isobutylene, styrene vinyl esters (including acrylic and methacrylie esters), etc., as well as condensation polymers such as organic polysulfide rubbers, including condensation products of ethylene dichloride or betabeta-dichlorodiethyl ether and other polysulfide condensation polymers.

The organic liquid component of the combustible mixture of the present invention is generally present as a plasticizer and may be any liquid compatible with the unvulcanized and vulcanized rubbery polymer used. It is preferably a liquid having a boiling point above C. Examples of suitable non-volatile liquids generally compatible with nonoil-resisting rubbery and rubber-forming polymers (including condensation products) are oil, including lubricating oils (and other high boiling hydrocarbons), styrene, and other organic polymerizable liquids, as well as liquid high boiling esters of one or more alcohols preferably with a higher alcohol (i.e. one with 4 or more carbon atoms) with an aliphatic, aromatic, or heterocyclic acid preferably with any organic dibasic acid, such for example as dioctyl phthalate, dioctyl sebacate, dibutyl phthalate, liquid polyesters such as esters of polyhydric alcohols and polybasic acids such as esters of ethylene, propylene, butylene, glycol, etc. with adipic, sebacic, and phthallic acids. For oil-resisting rubbery polymers which in the unvulcanized state are not swellable by hydrocarbons, it is generally desirable that the liquid constituent be any compatible plasticizer, such as an ester, such for example as those above named containing an aromatic group, a heterocyclic group, etc. It is essential in the operation of the present invention that the mixture of organic materials be fluid at one time after preparation of the mixture and that they be capable of changing state by the application of catalyst (including suitable vulcanizing agents and accelerators), and/or heat to the solid, rubbery or nonplastic state. It is usually more desirable that the rubbery material be capable of being vulcanized, i.e. subject to transition from the plastic or liquid state to the nonplastic state by the application of catalyst or suitable vulcanizing agents, heat or the like. Thus, rockets with suitable operational characteristics have been obtained when the rubbery constituent alone is capable of change from the plastic to the nonplastic state as, for example, by vulcanization after the vehicle has been cast to desired shape. As is well known, different vulcanizing agents are preferred for different rubbery polymers. Thus, peroxides are preferred for polysulfide rubbers, sulfur for Buna-type rubbers, and magnesia, etc. for neoprene.

In the preparation of a fuel for rockets embodying the present invention from a solid rubber, it is desirable in order to produce a vulcanizable liquid to add a suitable compatible high boiling liquid solvent to the rubbery component a little at a time while the rubbery component is being masticated by suitable apparatus, such as mill rolls or a dough-typemixer. Each addition of the liquid component should be added after the previous addition has been absorbed to form a compact mixture. The addition of the rubbery component to the liquid ingredient almost invariably causes formation of an inferior lumpy product. The inorganic oxidizing agent is preferably incorporated by stirring the pulverized solid directly into the fluid mixture. After the preparation of fluid and mixture of organic combustible material and inorganic oxidizing agent with the necessary amounts of solidifying agents present, the mixture is cast in a suitable mold, which may be a portion of the combustion chamber 2, or other suitable mold to provide a solid having a similar shape. The mold containing the fluid mix is usually treated, for example, by storing at an elevated temperature for an appropriate time to cause the organic fluid content to change from the liquid to the solid state, whereupon the fuel may be burned in the rocket to obtain the desired characteristics. The solid organic and/ or inorganic oxidizing agent is present in suflicient amounts to support vigorous combustion of the binding material and combustible ingredients of the casting. The amount of inorganic oxidizing agent is therefore at least equal in weight to the amount of organic materials present in the propellant. In general, the oxidizing agent should be present in suflicient amount to combine with substantially all of the carbon to form carbon monoxide therewith and, preferably, should also be present in sufficient amount to oxidize at least half the hydrogen present to water.

The mold containing the fluid mix is preferably incorporated in a zone of reduced pressure for a sufficient time to cause separation of most of the entrapped air and thus reduce tendency of formation of vacuoles within the rubbery fuel.

The casting mold may either be lined with an appropriate coating 7, or the coating 7 may be applied after the casting is solidified. When the fuel 4 is cast in place within the combustion chamber 2, the layer 7 is most conveniently applied directly to the walls of the fuel storage portion of the combustion chamber 2, as shown. When the fuel is cast in a separate mold, the coating may be most conveniently applied after the casting has hardened as by winding the casting with rubber or applying a vulcanized or unvulcanized rubber cover over the shaped castmg.

The following examples, in which parts are by weight, illustrate the preparation of rocket fuels in accordance with the present invention.

Example 1 Parts Neoprene KNR (polychloroprene) 300 Phenyl-beta-naphthylarnine 6 Zinc oxide Magnesium oxide l2 Tetraethylthiuramdisulfide 6 Piperidine pentamethylene dithiocarbamate 6 Arosol (a light aromatic base oil) 450 Potassium perchlorate 800 The phenyl-beta-naphthylamine and the magnesium oxide were incorporated into the neoprene while the latter was being masticated on a rubber mill. The partially plasticized neoprene was thereupon incorporated into a dough mixer of the Baker-Evans or Warner-Pfleiderer type, where the tetraethylthiuramdisulfide and the other accelerator were incorporated. The Arosol, which is an aromatic oil having an aniline point of about zero and which is produced by a large oil company, was then added a little at a time, successive additions being absorbed before the next addition was added to form a fluid mass having consistency of usual rubber cement. The oxidizing agent was thereupon added to the mixture and the mixing continued for a few minutes to provide a homogeneous material. The material 7 thus produced was cast in a mold having a shape and size corresponding to the fuel storage portion of the combustion chamber 2 of the rocket. The inside surfaces of the mold containing the fluid mixture were previously coated with a thick layer of solid asphalt or rubber and thereupon stored for twenty hours at atemperature of 180 F., whereupon solidification occurred without the formation of shrinkage racks of any kind.

Example 2 Parts Thiokol LP-2 Stearic acid 1 Zinc oxide -a 5 Hexamethylene tetramine l Tertiary butyl hydrogen peroxide 2.5 Potassium perchlorate 300 Thiokol LP2 is a vulcanizable polysulfide polymer consisting essentially of ethyleneoxymethyleneoxyethyleneoxy units joined by polysulfide linkages and having mercaptan terminal groups at the ends of the molecular chains. These mercaptan groups are readily converted by oxidation to cause the relatively fluid polymer to set to the vulcanized rubbery state. In mixing the above ingredients, the polymeric material is incorporated in a dough mixer of the Baker-Evans or Werner-Pfleiderer type, and the stearic acid, tertiary butyl hydrogen peroxide (curing agent), zinc oxide and hexamethylene tetramine are then added in any desired order and thoroughly mixed. While the mixture is maintained at about room temperature, the potassium perchlorate is mixed therein, a little at a time, until a uniform fluid mixture is obtained. The fluid mixture thus produced is cast in a mold having a shape and size corresponding to the fuel storage portion of a combustion chamber 2 of the rocket or jet, in the same manner as was the material in Example 1. The mold is allowed to stand at about 100 F. to F. until the material therein solidifies and the polymeric material is cured. solidification occurs without the formation of shrinkage cracks of any kind.

The polysulfide rubber of the above example may be substituted in whole or in part by other liquid polysulfide rubbery polymers or by fluid mixtures of polysulfide rubbers with compatible plasticizer, such for example as high boiling phthallic acid esters and the like.

The neoprene of Example 1 may be substituted by any one or more other vulcanizable rubbery materials compatible with lubricating oil in the unvulcanized state. The substantially inert oil which was used in the above examples to impart desired fluidity may be substituted by any one or more combustible liquids compatible with the rubbery polymer, or may be present in greater or lesser degree depending on the viscosity of the polymer. Certain synthetic rubbers may be prepared in the viscous or liquid state by changing the condensation or polymerization conditions during the formation thereof. These liquid rubbers may be vulcanized but need much less, if any, oil or plasticizer to provide the necessary fluidity.

The fuel portion of the rockets or jets of the present invention are generally characterized by an absence of plastic flow at temperatures of 200 F., and in many cases as high or higher than 250 or 300 F. They are also characterized by the absence of shrinkage cracks even when the rockets .or jets are maintained at temperatures as low as -40 F. Since the fuel has been cast from the liquid state where air bubbles may rise to the surface prior to solidification, there is little if any tendency for air bubbles to be retained in the fuel to cause, as do cracks and the like, the explosions heretofore frequently encountered with rockets containing solid fuels.

As used herein, the term polymers of a diolefinic compound is used in the generic sense to include copolymers as well as homopolymers of conjugated diolefinic compounds of less than 8 carbon atoms, such as dimethyl butadiene, isoprene and butadiene, as well as polychloroprene or neoprene. The term polysulfide rubber is used to designate the condensation polymers of alkaline polysulfides, such as sodium polysulfide with ethylene dichloride, beta-beta-dichlorodiethylether, and other organic compounds containing two substituents such as chlorine Which are split off by reaction with alkaline polysulfides. These and other examples of such polysulfide rubbers are described in one or more of the following United States Letters Patents of Joseph C. Patrick:

Re. 19,207 2,142,144 Re. 19,487 2,142,145 1,854,423 2,169,814 1,890,191 2,195,380 1,950,744 2,206,641 1,962,460 2,206,642 1,990,202 2,206,643 1,990,203 2,216,044 1,996,486 2,221,650 1,996,487 2,235,621 2,012,347 2,255,228 2,049,974 2,278,127 2,100,351 2,278,128 2,282,287

Although the invention may be employed in various ways, only preferred embodiments have been illustrated and described. Other embodiments may be made within the invention as provided by the patent statutes.

What I claim is:

1. The process of preparing a solid rocket fuel comprising preparing a compatible mixture of a solid inorganic oxidizing agent capable of supplying oxygen at elevated temperatures, and a liquid organic fuel constituent which requires externally supplied oxygen to support its combustion and which comprises an unvulcanized organic rubbery material selected from the group consisting of liquid polysulfide rubbers, solutions of polymers of diolefinic compounds of less than eight carbon atoms in high boiling hydrocarbons, and mixtures of polymers of diolefinic compounds of less than eight carbon atoms in high boiling esters of an alcohol with a dibasic organic acid, said oxidizing agent being present in amounts sufficient to supply oxygen to support vigorous combustion of the combustible ingredients of said organic fuel constituent without externally supplied oxygen, casting said rubbery material mixture into the desired shape, and curing said mixture of organic constituents in said fluid mixture, said oxidizing agent being present in an amount by Weight at least equal to the total weight of organic material present in the fuel.

2. The method of claim 1 wherein the rubbery material is a solution of a polymer of a diolefinic compound of less than eight carbon atoms in high boiling hydrocarbons.

3. The method of claim 1 wherein said rubbery material is a solution of a polymer of a diolefinic compound in liquid hydrocarbon.

4. A product produced according to the'process of claim 1.

5. A product produced according to claim 1 wherein the rubbery material is a peptized polysulfide rubber.

6. A product produced according to claim 1 in which the rubbery material is a solution of a polymer of a diolefinic compound of less than eight carbon atoms in a high boiling hydrocarbon.

7. A product produced according to claim 1 in which the rubbery material is a polysulfide rubber.

8. The method of claim 1 wherein the organic material comprises a mixture of rubbery polymer of a diolefinic compound and a liquid high boiling ester of a dicarboxylic organic acid.

9. The process of preparing a solid rocket fuel comprising preparing a fluid mixture of a solid inorganic oxidizing agent, capable of supplying oxygen at elevated temperatures, and at least one liquid organic fuel constituent, which requires externally supplied oxygen to support its combustion and which comprises an unvulcanized polysulfide rubber, said oxidizing agent being present in amounts sufficient to supply oxygen to support vigorous combustion of the combustible ingredients of said organic fuel constituent without externally supplied oxygen, forming said fluid mixture into the desired shape by introducing it into a mold, and curing said polysulfide rubber in said fluid mixture to form a solid fuel, said oxidizing agent being present in an amount by weight at least equal to the total weight of organic material present in the fuel.

References Cited UNITED STATES PATENTS 2,997,376 8/1961 Bartley 52.5

BENJAMIN R. PADGETT, Primary Examiner. 

